1. Field of the Invention
The present invention relates to a low-current-consuming switching regulator capable of quickly responding to variations in the output current and variations in the input voltage, and maintaining the output voltage at a predetermined voltage.
2. Description of the Related Art
An IC used for control operations (control-use IC) in a conventional switching regulator may degrade the system for supplying power from the power source due to a malfunction in control, in a generation transient state of the internal voltage of the IC resulting from the start/stop of the switching regulator. In a conventional switching regulator, in order to prevent such malfunction, a low voltage malfunction prevention circuit is used for forcibly turning off a switch element when the power source voltage drops below a specified voltage. In this case, as shown in FIG. 4, when the power source voltage becomes significantly low, and the internal power of the IC is not at a normal level, it cannot be accurately confirmed that the power source voltage has risen in a normal manner. Accordingly, the low voltage malfunction prevention circuit cannot operate normally.
Attempts have been made to solve this problem by providing two low voltage malfunction prevention circuits having different low voltage detection values (see, for example, patent document 1). One of the low voltage detection values is a value determined by characteristics of the power MOSFET used for the switch element, which is typically used in the conventional technology. The other low voltage detection value is for detecting whether the power voltage has become significantly low, which is determined by a threshold voltage of a group of transistors constituting the switching regulator. That is, when the power source voltage is lower than the threshold voltage of the group of transistors constituting the control-use IC, the switch element is forcibly turned off. With such a configuration, the low voltage malfunction prevention circuit operates even when the power source voltage is significantly low, so that the operation being performed in this state can be forcibly turned off, thereby preventing the above-described malfunction.    Patent Document 1: Japanese Laid-Open Patent Application No. 2005-78557
The above method is performed by detecting a state where the power source voltage is lower than the voltage determined by the threshold voltage of the group of transistors constituting the switching regulator, and turning off the power MOSFET constituting the switch element while such a state is being detected. However, when the power source voltage exceeds the threshold voltage of the group of transistors, the internal power of the IC may not necessarily have a normal voltage. When the internal power of the IC does not have a normal voltage, as shown in FIG. 5, there may be a period during which the circuit inside the IC including the low voltage malfunction prevention circuit becomes out-of-control. During such a period, a malfunction may occur, and the system may be degraded.